This is related to video distribution over a network including video conferencing. Video transmission uses equally sized data packets, each packet containing a complete video decodable unit. A minimal decodable unit for modern compression formats is a slice. A slice is a spatially distinct region of a frame that is encoded separately from any other region of the same frame. But hardware encoders usually operate at the granularity of a frame for performance reasons.
If the hardware encoder is not designed to support a slice size limitation then it is not possible to break the frame into slices, because macroblock (MB) distribution in slices is predefined in advance for the frame. Without slice size control (when hardware encoder is a black-box for software user), some compressed video slices may exceed the slice size limit.
The encoded slice size must not exceed a defined level, but splitting a frame into slices reduces encoding efficiency and requires a higher bit rate for the same visual quality. This visual quality can be achieved by iteratively re-encoding the frame with different slice partitioning until the required slice size reached.